60
10.10 Prepare the sample
To prepare the sample for testing:
1.
If the sample is turbid, either:
•
Filter the sample with a modified polysulfone filter
Before using other filter materials, test the filter material with 2% NaCl first to
make sure that the filter material can be used with the Luminescent Bacteria
Toxicity Test. Check the acceptable filters in the ISO method.
Note:
Do not use a cellulose nitrate or a cellulose acetate filter. The use of cellulose nitrate or
cellulose acetate filters can cause light inhibition that is not caused by the sample.
•
Let the sample sediment for 1 hour, or
•
Centrifuge the sample (e.g., 10 minutes at 5.000 g)
2.
Check the pH level. Adjust the sample to pH 6 to 8 using HCl or NaOH. Use a
strength of HCl or NaOH that does not change the volume of the sample by
more than 5% intotal.
3.
Add one spoon of solid NaCl (LCX058) and dissolve it in 7 mL of sample. The
concentration of salt in the test should not exceed 35 g/L.
Note:
Do not add NaCl to the sample if the salt concentration of the sample is more than 20
g/L (guide value: conductivity of 35 mS/cm).
Note:
The salt content of the sample should not exceed 50 g/L. This corresponds to a
conductivity of about 70 mS/cm without taking other conductive compounds into account.
Sodium chloride
A sodium chlorine (NaCl) concentrations of less than 15 g/L or
more than 50 g/L (or their osmolarity equivalents) in a sample will
cause osmosis-related light inhibition.
The addition of solid NaCl to the sample (2% final concentration),
prevents osmosis-related light inhibition of samples of low or
unknown NaCl concentrations.
Temperature
This biological test is strongly temperature-dependent.
ISO 11348 requires that the test is done under temperature
controlled conditions at 15 °C using a appropriate thermostat (i.e.
LUMIStherm, LTV053).
Turbidity and color
Causes high-bias results due to physical absorption or scattering of
light.
Use color correction cuvettes (accessories) in a separate test
according to ISO 11348 or dilute the samples (i.e. 25% or 50%)
before testing in the screening measure to remove the interference.
Interfering
substances
Interference levels and treatments
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